Re: base64 byte-length formula

["Julian Satran" <Julian_Satran@il.ibm.com>]

Sorry - It must have stayed in my pile. Your formula is not correct.
The correct one is (n*3 + 3)/4

Julo

Martins Krikis <mkrikis@yahoo.com> Sent by: owner-ips@ece.cmu.edu 06/13/2002 12:01 AM Please respond to Martins Krikis

To:        ips@ece.cmu.edu         cc:                 Subject:        base64 byte-length formula

I don't particularly care, but the formula given
on page 71 of 12-98 is wrong (and I've mentioned
it a few times already).

A very simple counterexample:

0 base64 digits should result in a 0-byte binary
string. According to the given formula, 
floor((0 + 3) * 3 / 4) - 0) = 2.

Another, more realistic but just as simple one:

4 base64 digits should result in a 3-byte binary
string. According to the given formula,
floor((4 + 3) * 3 / 4) - 0) = 5.

The given formula also fails for n = 3 (mod 4).

The correct formula, IMO is the following:
floor(n * 3 / 4).

In other words, there is no need to even count
the equivalence signs at the end, nor to add
any constants.

Martins Krikis, Intel Corp.

Disclaimer: these opinions are mine and may
            not be my employer's.


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